In a Long Term Evolution (LTE) system, there are three ways to reflect downlink physical Channel State Information (CSI): Channel Quality Indication (CQI), Pre-coding Matrix Indicator (PMI), Rank Indicator (RI).
The CQI is one indicator used for measuring downlink channel quality. The CQI is denoted by an integer value which represents different CQI grade respectively in a range of 1˜15 in 36-213 protocol, wherein the different CQI corresponds to its respective Modulation and Coding Scheme (MCS), and there are totally 16 conditions, which may be represented by 4 bits of information, as shown in TABLE 1.
PMI indicates that the User Equipment (UE) informs an eNode B (eNB) what a pre-coding matrix should be used to perform pre-coding on a PDSCH channel sent to the UE according to measured channel quality only in a transmission mode of closed loop space multiplexing, while RI is used for UE feeding back the number of layers for downlink transmission to eNB. A feedback granularity of PMI may be that the entire bandwidth feed back one PMI, or feed back PMI according to subband.
RI is used to describe the number of space separate channels, which corresponds to the rank of a channel response matrix. In a mode of open loop space multiplexing and closed loop space multiplexing, a UE is needed to feed back RI information, and there is no need to feed back RI information in other modes. The rank of the channel matrix corresponds to the number of layers of the channel matrix.
A transmission layer is a concept of multi-antenna “layer” in LTE and LTE-A, which denotes the number of valid separate channels in space multiplexing, and corresponds to antenna ports one by one in version 10, wherein an antenna port in version 10 is a logic port, and the total number of the transmission layers is RI. In addition in IEEE802.16m, a layer corresponds to a concept of “MIMO stream”, which has same physical meanings.
TABLE 1Relation between CQI index and MCSCQI indexModulation modeCoding rate × 1024efficiency0excess1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
In the LTE system, the feedback of CQI/PMI, RI may be a periodic feedback, or may be a non-periodic feedback, and the specific feedbacks are as shown in TABLE 2. Wherein, for periodic feedback CQI/PMI, RI, if UE needs not to send data, then the periodic feedback CQI/PM, RI are transmitted in a Physical Uplink Control Channel in a format of 2/2a/2b (in PUCCH format2/2a/2b), and if the UE needs to send data, then CQI/PMI, RI are transmitted in a Physical Uplink Shared Channel (PUSCH); and for non-periodical feedback CQI/PMI, RI, they are transmitted only in PUSCH.
TABLE 2Uplink physical channels corresponding to periodic feedbacksand non-periodic feedbacksPeriodic CQINon-periodic CQIScheduling modereport channelreport channelFrequencyPUCCHnon-selectiveFrequencyPUCCHPUSCHselective
In a LTE system, in different transmission modes, payload sizes of periodic CQI/PMI, RI transmitted in PUCCH channel are different, however, the biggest payload size is 11 bits. In a periodic feedback, a Reed-Muller (20, A) is adopted firstly to code M (M<=11) bits of CQI/PMI, RI to be fed back, and then the modulation is performed on coded bits, and they are transmitted in the form of PUCCH format 2/2a/2b.
As an evolution standard of LTE, the Long Term evolution Advanced (LTE-A) needs to support a greater system bandwidth (up to a maximum of 100 MHz), and an average spectral efficiency and a spectral efficiency of a cell edge user need to be enhanced, for this reason, the LTE-A system introduces many new technologies: (1) a downlink high-order Multiple Input Multiple Output (MIMO), the LTE system supports 4-antenna transmission at most in downlink, while an introduction of high-order MIMO make the LTE-A system support 8-antenna transmission at most in downlink, and then the dimensions of the channel state matrix increase; (2) Coordinated Multiple Point Transmission (CoMP), which is the coordinated transmission by utilizing multiple cell transmission antennas, the UE may need to feed back the channel state information of multiple cells.
By using multiple antennas at the sender (eNB), we may enhance transmission rate by way of space multiplexing, that is, different data are transmitted in different antenna locations in the same time-frequency resources of the sender, and by using multiple antennas at receiver (UE), we may distribute all of the antenna resources to the same user in the case of single user, and this transmission form is called as Single-User MIMO (SU-MIMO), in addition, we may also distribute resources of different antenna spaces to different users in the case of multiple users, and this transmission form is called as Multiple-User MIMO (MU-MIMO). In a single transmission mode, eNB may dynamically select the downlink SU-MIMO transmission or downlink MU-MIMO transmission according to reported channel state information, which we call as SU/MU MIMO dynamic switching.
A general process flow for dynamic switching transmission modes of single-user MIMO and multiple-user MIMO will be described as follows:
At first, a sender sends the pilot to user equipment in order for user equipment measuring downlink channel state, and the user equipment estimates the downlink channel according to the received pilot information, and the user equipment determines a format of feedback channel state information and reports the channel state information, and then the eNB selects a transmission mode of downlink SU-MIMO or a transmission mode of MU-MIMO dynamically according to the reported channel state information, and communicates according to the selected transmission mode.
For a dynamic switching transmission mode of single-user MIMO and multiple-user MIMO, on one hand, it keeps backward compatibility to support SU-MIMO as a priority principle and be compatible with a feedback form of CQI/PMI/RI of R8 as possible, on other hand, it needs to take into account forward compatibility to support MU-MIMO and COMP, ensuring that the new technology has acceptable performances. And there is a lack of mechanism for reporting overall and effective channel state information in a SU-MIMO and MU-MIMO dynamic switching system in the existing technology, causing that the eNB can not correctly select a downlink channel used when sending data to the UE, thereby causing that a fast switching of two transmission forms of SU-MIMO and MU-MIMO is not supported, reducing the work efficiency of the system.